The present invention relates to a process for the production of a refrigerating circuit comprising non-evaporable getter material for removing gases, particularly atmospheric gases, from the fluid mixture contained in refrigerating circuits for refrigerators and cooling devices in general.
It is well known that the most common cooling system is based on the physical principle of drop of temperature of a fluid during its evaporation and is employed in domestic or industrial refrigerators, freezers, automatic dispensers of perishable foodstuffs, refrigerated shop-windows, air conditioners, etc. This principle is applied by using closed circuits containing a fluid suitable to be subjected to compression and expansion cycles. The circuit, comprising a compressor, extends mainly with a very small, substantially capillary cross-section, being coil shaped in order to increase the surface available for the exchange of heat, and is normally made of copper, an excellent heat conductor. A molecular sieve filter is generally provided upstream of the coil and the tubular portion of the evaporator with a larger cross-section lies downstream thereof, before the return to the compressor. Usually this is the general configuration, apart from possible variations.
The fluid is selected among those undergoing liquid-vapor phase transition caused by pressure changes in the temperature range of 0-50xc2x0 C. During the expansion step, a partial evaporation of the liquid occurs, causing its temperature to drop, and heat is removed from the parts to be cooled through the closed circuit metal walls; during the compression step, the previously formed vapor condenses, thus releasing heat that is transferred outside of the system. As cooling fluids chlorofluorocarbons (CFCs) were previously used, but their industrial use has been forbidden because of their reaction with ozone in the upper part of the atmosphere. Hydrogenated CFCs (HCFCs) are used as substitutes, and the use of lower saturated hydrocarbons, such as isobutane ((CH3)3CH), is spreading. These compounds are generally used in admixture with oils, ensuring the continuous presence of a liquid phase for the correct working and lubrication of the mechanical parts of the compressor. In the following the cooling oil-fluid mixture will be simply referred to as freezing mixture.
The presence, in the pipes composing the closed refrigerating circuit, of gases other than the working fluid vapors, generally atmospheric gases, causes some problems. First, these gases are not condensable by compression at the typical compressor working temperatures (around room temperature), and as a result remain in the circuit as gases. Because of their compressibility, part of the compression/expansion work done by the compressor is transformed into a simple elastic variation of their volume and does not contribute to the evaporation/condensation cycle accomplishing the heat transfer, with the net result of a decrease of the compressor energetic yield. Moreover, the presence of gases in the refrigerating circuit causes noises, annoying especially in the case of domestic refrigerators. Finally, when the cooling fluid is a hydrocarbon, the presence of air involves a certain risk of explosions that, however remote, still is not negligible.
The production of refrigerating closed circuits comprises a step of evacuation of the metallic pipes by mechanical pumping, in order to remove most of the initially contained air, and the successive filling of the circuit with the oil/cooling fluid mixture. However, the normal evacuation operations carried out industrially do not allow a complete gas removal, such as to eliminate the above-described difficulties. A complete evacuation would require long pumping times, unacceptable for industrial applications.
Italian patent application MI 98A 000558 in the name of the same applicant aims at providing a getter system comprising a getter material held within an evacuated chamber having at least one wall contacting the freezing mixture inside the circuit. The wall is made of a material permeable to the gases but not to the fluids constituting the mixture itself.
In this way the non-evaporable getter material sorbs the atmospheric gases, which are present in the cooling fluids during the circuit working life, as soon as the fluid contacts the getter material, in spite of the reduced conductance values of the circuit itself. This results in long times being necessary for sorbing the gases left in the circuit as residues from the production process. The getter material is therefore used as in the high vacuum systems, but these circuits are never under a very high vacuum, and the degassing problem is negligible compared to the advantage of having, already at the start of the operation, the greatest reduction of unwanted gases present in the circuit.
U.S. Pat. No. 5,718,119 discloses methods for eliminating air from a refrigerating circuit during the manufacturing steps thereof. A first method consists in connecting to the refrigerating circuit an air absorbing device containing zeolites, allowing the device time for absorbing air, and then disconnecting the air absorbing device from the circuit prior to its backfilling with the refrigerating fluid. A second method consists in replacing air in the circuit with carbon dioxide (CO2), connecting to the circuit a CO2 sorbing device containing zeolites, calcium hydroxide and calcium chloride, or an epoxy compound, so as to absorb CO2 from the circuit, and then disconnecting the CO2 absorbing device from the circuit prior to its backfilling with the refrigerating fluid. The methods disclosed in this patent are rather complex, in that absorption of atmospheric gases by zeolites requires the use of a supplemental refrigerating unit to cool these down to very low temperatures; the use of the second method requires an additional step of filling with CO2 followed by its removal.
European published patent application EP-A-0 633 420 discloses jackets whose thermal properties may switch from thermally insulating to thermally conducting; the change of condition of the jacket is based on a mechanism of absorption/desorption of hydrogen from materials, generally zirconium-based alloys, showing hydrogen sorption properties that are reversible depending on operation temperature.
The above-mentioned evacuation is obtained according to the present invention without these inconveniences of the prior art by a process for the production of a refrigerating circuit comprising introducing non-evaporable getter material into a refrigerating circuit, evacuating the circuit by pumping, and heating the getter material at a temperature of at least 200xc2x0 C. during the evacuation or in an immediately subsequent step.
An object of the invention is also a refrigerating circuit made by this process, as well as any apparatus containing such a circuit.